Method for operating a gas and steam turbine installation for frequency support

ABSTRACT

A method for operating a gas and steam turbine system having a gas turbine, a steam turbine and a waste heat steam generator is provided herein, wherein steam for the steam turbine can be generated in the exchange of heat with exhaust gas from the gas turbine. Absorption capacity of the steam turbine can be increased and pressure in the waste heat steam generator can be lowered to utilize storage reserves in the waste heat steam generator for increased generation of steam to assist the frequency in the power system starting from a steady-state operating mode. Thermal energy is fed to the waste heat steam generator wherein a power profile of the gas and steam turbine system is greater than or equal to a preceding power level of the steady-state operating mode to increase the absorption capacity of the steam turbine and reduce pressure in the waste heat steam generator.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International Application No. PCT/EP2012/071478 filed Oct. 30, 2012, and claims the benefit thereof. The International Application claims the benefit of European Application No. EP11188956 filed Nov. 14, 2011. All of the applications are incorporated by reference herein in their entirety.

FIELD OF INVENTION

The invention relates to the frequency support operation of a gas and steam turbine installation.

BACKGROUND OF INVENTION

The energy market increasingly requires highly flexible power station installations which, in addition to short startup and shutdown times, can also cover a large power range and are well prepared for frequency support operation.

This also includes, inter alia, the ability to free up additional power in the event of high electricity demand (what is termed peak load operation). In this context it will be expected in the future that power stations which are operated at their setpoint also participate in peak load coverage and frequency support.

Present solutions focus on the use of power reserves within the components or rely on technologies which can make only a very small power reserve available. Both for frequency support and for peak load coverage, the gas turbine can be overfired, the compressor guide vanes can be opened beyond the base load setting, or water can be injected into the intake air duct. Requirements which relate only to peak load coverage can be satisfied by injecting steam into the gas turbine combustor, by cooling the gas turbine intake air, for example by means of evaporative coolers or using refrigeration machines (what are termed chillers), or by equipping the waste heat steam generator (WHSG) with an auxiliary firing means in order to raise the steam turbine power. For the purpose of frequency support, the fresh steam or the steam from the reheating (modified sliding-pressure operation) can be accumulated and the turbine control valves can subsequently be opened quickly. EP 1 164 254 B1 describes a gas and steam turbine installation having steam bypass lines for peak load coverage, i.e. for auxiliary power at full load. In that context, part of the steam generated in the waste heat steam generator is fed via bypass ducts past the turbine inlets and into further inlets, arranged downstream of these turbine inlets, to the turbine parts, whereby the pressure in the waste heat steam generator can be kept substantially constant and the absorption capacity of the steam turbine—and thereby also the power output—can be increased.

SUMMARY OF INVENTION

It is an object of the invention to provide a method for the frequency support operation of a gas and steam turbine installation, which method makes an improved power reserve available.

Embodiments of the invention achieve this object by providing that, during the operation of a gas and steam turbine installation having a gas turbine, a steam turbine and a waste heat steam generator, in which steam for the steam turbine can be generated by heat exchange with exhaust gas from the gas turbine, for frequency support in the power grid, starting from steady-state operation, the absorption capacity of the steam turbine is increased and the pressure in the waste heat steam generator is decreased in order to use storage reserves in the waste heat steam generator for increased steam generation, and that heat energy is supplied to the waste heat steam generator at such a rate that, as a consequence of increasing the absorption capacity of the steam turbine and decreasing the pressure in the waste heat steam generator, a power profile of the gas and steam turbine installation is greater than or equal to an immediately previously available power of the steady-state operation.

Embodiments of the invention are accordingly based on the idea of using storage reserves in the waste heat steam generator in order to generate additional steam by abruptly opening the valves. As a consequence of the pressure drop in the waste heat steam generator, steam is in addition generated and an adequately dimensioned and rapid supply of heat energy should prevent the common dip in the power profile. By means of this method, control power can be provided at partial load and full load.

By means of a method according to an embodiment of the invention, the flexibility and profitability of the power station installation can be substantially increased, since at high power demand additional energy is available which, in particular through high electricity sales in electricity markets, leads to increased revenue and makes the operation of the installation more profitable (peak load ability). This holds for frequency support operation, in particular for secondary and tertiary support. It is thus not necessary for primary frequency support or peak load operation to configure the high-pressure part and also the reheating part with higher pressure than for rated operation. In addition, it is not necessary to run the installation in what is termed modified sliding-pressure operation which, as a consequence of throttling back the steam turbine control valves, causes losses in power and efficiency during standby operation of the installation. By means of the method according to an embodiment of the invention, the load range of the power station can be extended, as the low load operation can also be adjusted more flexibly.

Advantageously, in order to increase the absorption capacity of the steam turbine, at least one valve in a bypass duct is opened in order to bypass a steam turbine stage or a steam turbine module.

In this context, it is expedient if steam is fed via the bypass duct into the steam turbine downstream of a high-pressure intake.

It is particularly advantageous if, alternatively or additionally, steam is fed via the bypass duct into the steam turbine downstream of an intermediate-pressure intake.

Alternatively or complementarily, it can be advantageous if, in order to increase the absorption capacity of the steam turbine, at least one valve of a control wheel on a high-pressure turbine and/or an intermediate-pressure turbine is opened.

Preferably, the heat energy is supplied by means of increased power from the gas turbine and thus an increased flow of exhaust gas.

It can furthermore be advantageous if the heat energy is supplied by an auxiliary firing means. This must, however, be dimensioned accordingly.

In order to further increase the storage capacity, it is expedient if, during steady-state operation, a steam drum pressure is accumulated by means of a valve which is opened for frequency support.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail by way of example with reference to the drawings, which are schematic and not to scale, and in which:

FIG. 1 is a simplified circuit diagram of a gas and steam turbine installation having high- and intermediate-pressure overload introduction and control wheels in the steam turbine and an auxiliary firing means in the waste heat steam generator,

FIG. 2 shows a steam turbine power profile for overload introduction into the high-pressure turbine for various fresh steam pressure to introduction pressure ratios and

FIG. 3 shows a steam turbine power profile for overload introduction into the intermediate-pressure turbine for various fresh steam pressure to introduction pressure ratios.

DETAILED DESCRIPTION OF INVENTION

FIG. 1 shows a gas and steam turbine installation 1 which comprises a gas turbine 2 and a steam turbine 3. A rotor of the gas turbine, a rotor of a generator 5 and a rotor of the steam turbine 3 are coupled to one another by means of a shaft 4, wherein the rotor of the steam turbine 3 and the rotor of the generator 5 can be rotationally separated from one another and coupled to one another by means of a coupling 6. The rotors of the generator 5 and of the gas turbine 2 are fixedly interconnected. A flue gas outlet from the gas turbine 2 is connected, by means of an exhaust gas line 7, to a waste heat steam generator 8 which is provided for generating the working steam of the steam turbine 3 using waste heat from the gas turbine.

When the gas and steam turbine installation 1 is in operation, the rotating rotor of the gas turbine 2 drives, via the shaft 4, a compressor 9 by means of which combustion air is drawn in from the surroundings and fed to a combustor 10. There, the combustion air is mixed with fuel supplied by a fuel feed 11 and is burnt, and the hot, pressurized exhaust gases are fed to the gas turbine 12 where they are expanded, performing work. The exhaust gases, still at a temperature of 500 to 600° C., are then fed via the exhaust gas line 7 to the waste heat steam generator 8 which they flow through until they pass into the environment by way of a chimney 13. On their path through the waste heat steam generator 8, they supply their heat to a high-pressure superheater 14, then to a high-pressure reheater 15, to a high-pressure evaporator 16, to a high-pressure preheater 17, then to an intermediate-pressure superheater 18, to an intermediate-pressure evaporator 19, to an intermediate-pressure preheater 20, then to a low-pressure superheater 21, to a low-pressure evaporator 22 and finally to a condensate preheater 23.

Steam superheated in the high-pressure superheater 14 is fed through a steam takeoff line 24 to a high-pressure stage 25 of the steam turbine 3 where it is expanded, performing work. With the work—as with the work performed in the gas turbine—the shaft 4 and thus the generator 5 are moved to generate electrical energy. The hot steam partially expanded in the high-pressure stage 25 is then fed to the high-pressure reheater 15 where it is once again heated, and is fed through a takeoff line 26 to an intermediate-pressure stage 27 of the steam turbine 3 where it is expanded, performing mechanical work. The steam partially expanded there is fed via an overflow line 28 to a low-pressure stage 29 of the steam turbine 3 where it is further expanded, providing mechanical energy.

The expanded steam is condensed in the condenser 30 of the steam turbine 3 and the resulting condensate is fed by means of a condensate pump 31 directly to a low-pressure stage 32 of the waste heat steam generator 8, or by means of a feed pump 33—by which it is provided with appropriate pressure—to an intermediate-pressure stage 34 or to a high-pressure stage 35 of the waste heat steam generator 8, where the condensate is evaporated. After steam generation and superheating, the steam is fed via the relevant takeoff lines 24, 26, 36 of the waste heat steam generator 8 back to the steam turbine 3 to be expanded and to perform mechanical work.

Shut-off fittings 37 and 38 are arranged in the steam takeoff lines 24 and 26. A bypass duct 39 having a shut-off fitting 40 for bypassing the high-pressure stage 25 branches off from the steam takeoff line 24 leading to the high-pressure stage 25 of the steam turbine 3. A bypass duct 41 having a shut-off fitting 42 for bypassing the intermediate-pressure stage 27 branches off in similar fashion.

A first control wheel 43 is attached to the rotor of the steam turbine 3 upstream of the high-pressure part 25 as seen in the flow direction. A second control wheel 44 is similarly attached to the rotor of the steam turbine 3 upstream of the intermediate-pressure part 27 as seen in the flow direction. A control wheel comprises nozzles which are controlled by valves and by means of which in each case segments of a turbine can be acted upon. Depending on how many valves are open, a greater or lesser quantity of auxiliary steam flows via the nozzles into the turbine.

Furthermore, FIG. 1 shows, at the entrance to the waste heat steam generator 8, an auxiliary firing means 45 in which fuel is added to the gas turbine exhaust gas, which still contains a lot of oxygen, and the mixture is burnt. In this way, the fresh steam can be heated above the temperature of the gas turbine exhaust gas or for generating process steam if the steam generation is to be decoupled from the electricity generation of the gas turbine 2. In particular, an auxiliary firing means 45 can be of interest in order to increase the output of electrical power at peak demand times.

The method according to an embodiment of the invention provides that the mass flow rate of steam through the steam turbine is increased at short notice by opening an overload valve 40, 42 or a turbine bypass 39, 41 and, in connection therewith, the power of the steam turbine 3 rises rapidly (in seconds).

The overload introduction may, according to an embodiment of the invention, be used both on the high-pressure turbine 25 for raising the mass flow rate of fresh steam and on the intermediate-pressure turbine 27 for raising the mass flow rate of reheat steam, and also upstream of each further turbine stage (e.g. low-pressure turbine 29).

As an alternative, the absorption capacity of the steam turbine can be increased by means of a control wheel 43, 44 on the high-pressure turbine 25, and/or on the intermediate-pressure turbine 27, by opening corresponding valves.

In this context, storage reserves from all pressure stages 32, 34, 35 of the waste heat steam generator 8 (e.g. also intermediate- and low-pressure system, where present) can be released. By accumulating the drum pressure, e.g. by means of a pressure regulating valve 46 in the intermediate-pressure steam system 34, the storage capacity can in this context be increased. This increase in the mass flow rate of steam is based on an increase in the absorption capacity of the steam turbine and, associated therewith, a drop in pressure in the system.

This drop in pressure leads to a storing of thermal reserves (hot water, steel masses in the waste heat steam generator) and thus to a short-term increase in power of the steam turbine, as FIGS. 2 and 3 show for the power profile in the event of overload introduction for various ratios of fresh steam pressure to introduction pressure in the high- or intermediate-pressure turbine. The horizontal line indicates the value for steady-state operation.

Since the thermal storage reserves are limited, the decreasing storage effect is compensated for or further increased, according to an embodiment of the invention, either by a self-igniting auxiliary firing means 45 in the waste heat steam generator 8, an auxiliary firing means 45 operating at a continuous minimum load or by means of available power reserves in the gas turbine 2 (opening the compressor guide vanes, overfiring, steam injection or water injection into the compressor 9 or combustor 10). 

1. A method for operating a gas and steam turbine installation, having a gas turbine, a steam turbine and a waste heat steam generator, in which steam for the steam turbine can be generated by heat exchange with exhaust gas from the gas turbine, wherein the method comprises, for frequency support in the power grid, starting from steady-state operation, increasing the absorption capacity of the steam turbine and decreasing the pressure in the waste heat steam generator in order to use storage reserves in the waste heat steam generator for increased steam generation, and supplying heat energy to the waste heat steam generator at such a rate that, as a consequence of increasing the absorption capacity of the steam turbine and decreasing the pressure in the waste heat steam generator, a power profile of the gas and steam turbine installation is greater than or equal to an immediately previously available power of the steady-state operation.
 2. The method as claimed in claim 1, wherein, in order to increase the absorption capacity of the steam turbine, at least one valve in a bypass duct is opened in order to bypass a steam turbine stage or a steam turbine module.
 3. The method as claimed in claim 2, wherein steam is fed via the bypass duct into the steam turbine downstream of a high-pressure intake.
 4. The method as claimed in claim 2, wherein steam is fed via the bypass duct into the steam turbine downstream of an intermediate- pressure intake.
 5. The method as claimed in claim 1, wherein, in order to increase the absorption capacity of the steam turbine, at least one valve of a control wheel on a high-pressure turbine and/or an intermediate-pressure turbine is opened.
 6. The method as claimed in claim 1, wherein the heat energy is supplied by means of increased power from the gas turbine and thus an increased flow of exhaust gas.
 7. The method as claimed in claim 1, wherein the heat energy is supplied by an auxiliary firing means.
 8. The method as claimed in claim 1, wherein, during steady-state operation, a steam drum pressure is accumulated by means of a valve which is opened for frequency support. 